An AOTF is essentially a solid-state agile random-access tunable filter, where the wavelength is selected by an RF drive signal applied to an electrode attached to an acoustic transducer that is attached to a birefringent interaction medium, such as a tellurium dioxide (TeO2) crystal. It is well known that AOTFs provide one way of realizing a fast hyperspectral imager, since the AOTF can rapidly switch between wavelength bands. However, if the AOTF is placed in the optical train within a collimated beam (“afocal system”, i.e., no intermediate image formed in the AOTF) a particular form of aberration peculiar to the AOTF known as “acousto-optic blur” or “acoustic blur” causes the instrument point spread function (PSF) to increase in size in the direction corresponding to the acoustic scattering plane.
Thus, an optical instrument with its AOTF removed and replaced with an equivalent fixed bandpass filter centered at the same wavelength would exhibit a PSF in the form of a blurred circle or disc when used to image a pointlike object which is below the instrument's resolution limit. This is normal behavior for a conventional optical system having rotational symmetry. If the AOTF is replaced in a collimated space between the input conditioning optics and a photodetector such as a focal plane array (FPA), making suitable adjustments for the wanted diffracted beam exiting from the AOTF being deflected by a small angle from the unwanted (“zero-order” undiffracted) beam, then the situation changes. The diameter of the PSF will be substantially unchanged in the direction orthogonal to the scattering plane, but will increase in the direction corresponding to the plane of scattering, thus making the PSF appear elliptical.
The amount of “elongation” depends on the optical bandwidth of the AOTF. This effect is caused because the diffracted (first-order) rays exiting the AOTF have a direction which depends weakly on the actual wavelength. As the AOTF has a finite bandwidth, which may be a few nanometers (typically), rays with wavelengths falling in the range at which transmission occurs will have a small but finite angular spread (e.g., tens of micro-radians). These rays appear to emanate from a region inside the AOTF if traced back. If the system is afocal, then at the detector array (e.g., FPA), objects in focus are will be somewhere in front of the input conditioning lens, and the bundle of rays corresponding to a pointlike distant object will be parallel (i.e., form a collimated bundle of rays or beam). Thus, if another object is placed in the AOTF, it will appear out of focus, thus accounting for the small halo of “blur” which this optical configuration produces.
This blurring effect can be minimized by increasing the length of the acoustic transducer to increase the interaction length, that in turn reduces the filter bandwidth, which minimizes the angular spread of the diffracted rays corresponding to the pointlike object. Instead of a “pointlike object”, one can imagine concentrating only on the rays emanating from a small area of a finite sized object, with this area tending to a very small region, and tracing their passage through the AOTF.
The blurring results because the AOTF is trying to “look at” two objects (the distant real one and the local “AOTF-induced” object) at different distances and bring them to focus at the detector array simultaneously. However, this is impossible since one or both of these objects will be out of focus at any particular setting of the optics, so that both objects will never be capable of being focused simultaneously.
One solution to this problem is to form an intermediate image of the “object point” inside the AOTF at the location where the diffracted, filtered rays appear to come from. When this is done, the angular spread due to acoustic blur and the angular spread due to the object relayed into the AOTF are two fans emanating from the same point, and thus cannot be distinguished. This arrangement eliminates the need for the AOTF to focus on two differently spaced objects at the same time.
This solution is well known, and one realization is shown as a schematic diagram in FIG. 1 referred to as a telecentric-confocal AOTF-based imaging system 100. Refraction at the input and exit surfaces of the AOTF is ignored in FIG. 1 for simplicity. The term “telecentric” is known in optics and refers in this case to an aperture stop (S1) positioned at the input and a beam stop (S2) positioned at the output of the system 100 identified as an “exit pupil” at a distance from their respective lenses 101 and 102 equal to the focal lengths f1 and f2 associated with their respective lenses. Stops S1 and S2 are shown at conjugate positions (i.e., so that the image of the aperture stop S1 is formed at the beam stop S2).
The aperture stop S1 ensures that the optical setup is telecentric for the object space, so that the chief ray of each ray pencil is incident on the AOTF 120, that is shown having an acoustic transducer 125 thereon, at normal incidence, or at least at the same angle. The position of the beam stop S2 at the front focal length f2 of lens 102 efficiently blocks the unwanted zero order unfiltered light transmitted by the AOTF 120 to ensure telecentricity in the image space. This is the light that passes straight through the AOTF 120 because it contains wavelengths outside the bandwidth of the AOTF 120. Since the zero-order beam is generally far brighter than the wanted filtered beam, removing it efficiently is important. System 100 also includes a photodetector, such as the camera 160 shown.
System 100 causes a slight, but generally insignificant, variation of the center wavelength of the AOTF 120 with position. Moreover, it can be seen that the total length of system 100 between the aperture stop S1 and beam stop S2 is equal 2f1+2f2.